This invention addresses the problem of variations in the tranverse width of a plastic film web in a gusseting process. A shortening of the transverse width of the web can result in a severing or tearing of the web or misalignment and lack of symmetry among the layers of the web.
Gusseted plastic bags are typically manufactured from a substantially continuous sheet of plastic film folded in half to form a multilayer film web. Each layer of film may comprise one or more plies of plastic or plastic compatible material. The edge of the film web opposite the folded-over edge portion may be open-ended or closed by fastening means such as a zipper.
A gusset is formed when the folded-over edge portion is folded inward into the film web to form gusset folds along the affected film web fold portion. Numerous gusset fold configurations including bi-fold and tri-fold are possible. The film web may then be heat sealed to form individual gusseted bags. Upon filling of the gusseted bag, the gusset folds expand to reveal side or bottom walls of dimension equivalent to the width of the folds when opened. Gusseted plastic bags have advantages over ungusseted plastic bags in that they may stand upright upon filling.
Numerous gusseting apparatuses are known in the prior art, including Piazze, U.S. Pat. No. 3,618,478, which relates to the gusseting of an open-ended web. A typical apparatus usually comprises a pair of generally aligned, closely-spaced plates positioned inside the film web contiguous to the interior face of folded-over web edge portion, with another plate, preferably in the shape of a wheel, positioned a certain distance therebetween from the external face of the folded-over web edge portion inward.
Variations in the transverse width of the web can result in misalignment of the web or the layers of the web with respect to each other. This can result in tearing or excessive variation in the depth of the bag or lack of symmetry among the layers of the bag or the gusset folds. If the web has a zipper, misalignment may result in difficulties in separation, closure, and alignment of the rib and groove profiles of the zipper.
Variations in transverse width are more prevalent in both number and magnitude in film webs produced by blown extrusion processes than film produced in cast extrusion processes, but are present to a certain degree in all film webs.
A representative prior art gusseting apparatus which compensates for variations in the transverse width of the web is related by Wech, U.S. Pat. No. 3,618,813. Wech relates a resiliently-biased wheel positioned a certain distance between two plates with the preferred means of resilience being a spring. When tension exerted on the film by the plates and the wheel positioned therebetween becomes greater than the compressive force of the biasing spring, the wheel is forced backwards a distance sufficient to equalize the tension and the compressive force.
The shortcomings of spring-biased gusseting wheels known in the prior art include difficulty of controlling and selecting the desired tension level of the wheel, variation in spring force as a function of spring displacement, and the tendency of the spring to bias continually or continuously or "wave" around the tension-spring force equilibrium point during the gusseting operation.
An element of uncertainty is involved in setting a spring-biased gusseting wheel at uniform preload; the desired tension must be manually set by manipulation of a mechanical adjusting device on the basis of visual observation of the depth of the gusset.
Variation in spring force as a function of spring displacement is critical since spring displacement is necessary to relieve tension between the film and the protruding plate. If, upon compression or stretching of the spring, the spring force increases significantly over the displacement, then the spring, and, thus, the protruding plate or gusseting wheel, may not bias to a sufficient degree to adequately relieve tension between the film and the protruding plate. If the tension cannot be relieved, then a severing or puncturing of the film by the edge of the protruding plate or misalignment of the layers of the film web is possible. At the very least, variation in spring force creates uncertainty as to determination of desirable gusset depths for a given spring utilized in the gusseting apparatus.
The tendency of the spring to "wave" around the tension-spring force equilibrium point during the gusseting operation is the most critical problem associated with the spring-biased protruding plate. Waving of the spring results from a lack of intrinsic dampening characteristics in the system.
The phenomena of waving arises as follows: the protruding plate or gusset wheel biases in reaction to a change in the transverse width of the web, the biasing motion has an intrinsic momentum which creates an alternating tensile stretching and compression motion within the spring that dissipates according to intrinsic dampening characteristics of the gusseting apparatus and the spring itself. The problem with relying upon the intrinsic dampening ability of the spring to dissipate waving is that variations in transverse width of the film web arise often enough to create continuous or at least very frequent continual biasing or waving of the protruding plate or gusset wheel.
Process problems and difficulties brought on by waving include excessive variation in bag and gusset depth and lack of symmetry among the gusset folds.
Variation in bag and gusset depth along lengths of film web due to waving occurs because the spring will wave or bias around any given tension-spring force equilibrium point whether there has been a change in the transverse width of the film web or not.
Lack of symmetry among the gusset folds due to waving occurs when the gusseting wheel or protruding plate is on its outward motion. The temporary excessive slack in the film web can become unevenly distributed between the receiving plates resulting in adjacent gusset folds having differing depths with respect to each other.